This invention relates to electrodeposition and more particularly to the electrodeposition of refractory metal silicides.
The advantages which greater hardness in metals offers is well recognized. Harder metals, more resistant to wear, reduce the need for frequent and costly replacement of parts.
Abrasive wear afflicts all manner of machinery in which metal surfaces contact other surfaces. For example, erosive wear plagues metals exposed to high velocity gas streams carrying hard particles, as in coal gasification, or even the lower velocity, liquid-entrained coal particles in a slurry flowing through a pipeline. The wearing of metals is frequently aggravated by high temperatures which lead to simultaneous metal oxidation, particularly in the newer energy industries.
Several approaches to reducing wear have been taken. Chief among these has been the formulation of ever harder alloys, such as the newer ones based on cobalt. Another route has been to modify only surface properties, rather than the bulk of the metal. This has been done by covering the bulk metal with a coating of another alloy. Still another method has been to modify the surface layer of the metal either by diffusing other metals into the surface (metalliding), by ion implantation, or by laser melting.
It has long been recognized that refractory silicides possess precisely the desirable hardness missing from metals and are stable at high temperatures. Nevertheless, such refractory silicides lack the desirable ductility of metals. Consequently there have been many attempts to combine the two in order to gain hardness combined with ductility. One approach has been to produce silicide coatings on metals. However, existing coating methods have not been entirely successful. Plasma spraying, which involves impinging the silicide powder on the surface to be coated, requires temperatures near 1500.degree. C., is line-of-sight, and tends to produce somewhat porous coatings. Chemical vapor deposition can be carried out by combining two reactive gases so that the silicide reaction product is produced as a coating. Much development work has been done on this process, but the coatings are usually quite thin. Further, neither plasma spraying nor chemical vapor deposition allows any control over the stoichiometry of the coating.
During the 1960's, Senderoff and Mellors in "Coherent Coatings of Refractory Metals" Science (1966) volume 153, pages 1475-1481, incorporated herein by reference, showed that excellent coatings of the refractory metals could be electroplated from the ternary eutectic of (Li, Na, K) F by adding the metal as a complex fluoride, and plating between the appropriate metal anode and the cathode to be plated at 750.degree. C.-800.degree. C. Dense, adherent, and ductile plates were obtained, and there seemed to be no upper limit to the plating thickness; in fact, the substrate could be dissolved away to produce freestanding refactory metal objects.
Refractory metal carbide coatings have been electrodeposited as disclosed in U.S. Pat. No. 4,430,170, to Stern. Hard adherent coatings of any desired thickness were formed. The process comprises adding the refractory metal as a complex fluoride and the carbon as an alkali carbonate to an alkali fluoride melt. An anode comprised of the refractory metal and a cathode comprised of the article to be coated are immersed in the melt. When a voltage is applied across the cathode and anode the carbon and metal cations are simultaneously reduced at the cathode to form a refractory metal carbide coating.
Silicide coatings on metal articles have been formed by metalliding as disclosed in U.S. Pat. No. Re. 25,630 to Cook. In the metalliding process, silicon is added to a fused complex metal salt bath as silicofluoride. The silicon is dissolved in the bath and the metal article to be coated is immersed in the bath. The silicon diffuses into the metal and reacts with the metal to form a silicide coating. This process forms a non uniform coating wherein the concentration of silicon is the highest at the surface of the metal. In addition the rate at which the silicon diffuses into the metal decreases with time the result of which is that the process slows down as the thickness of the coating increases.